The present invention relates in general to the art of oil recovery and, more in particular, to recovery of hydrocarbons from heavy crudes or bitumens by stimulation.
There are large petroleum deposits in the form of very viscous crudes or bitumens. These deposits may be residuals from naturally developed fields or deposits which have never been produced. An example of very viscous tar deposits is in the Peace river and Athabasca regions of Canada. These tars have a gravity of from 6.degree. to 20.degree. API, a clean oil viscosity of to 20,000 cps, and an emulsion viscosity of to more than 100,000 cps. The asphaltene content of these deposits is up to 30% and sulfur up to 6%. Because these tars are so viscous, they cannot be recovered by natural techniques and must be stimulated.
Stimulation of petroleum deposits by steam flooding is a known and tested technique. In this type of stimulation, high pressure and temperature steam is injected into injection wells for recovery of petroleum from production wells. During steam stimulation, steam heats a deposit in a steam zone. Values are distilled there and are forced by steam pressure away from the injection wells towards the production wells. Some of the distilled hydrocarbons will condense in the steam zone because of heat loss from the zone to surrounding strata. Some of the distilled hydrocarbons will reach a front between a hot condensate zone and the steam zone and condense there. The driving force of the steam pressure, however, continuously advances the condensed hydrocarbons towards the production wells. The hot condensate zone itself fronts on a cold water zone more remote from the steam zone. Finally, there is an oil zone bordering the cold water zone which is the formation unaffected by stimulation. In typical steam flooding, the cold water zone is water flooded and oil is removed by this known technique to the water flooding saturation level. The advancement of the hot condensate zone itself stimulates recovery by lowering viscosity of the oil and by thermal expansion of the oil. Within the steam zone, recovery is promoted, in addition to distillation, by the temperature produced agencies of viscosity reduction and formation swelling. Hydrocarbons are usually recovered at the production wells in primarily liquid form. The considerable driving force of the steam flooding technique is ultimately lost when breakthrough occurs at a production well. This is an event where the steam front advances to the production well and steam pressure is largely dissipated in the well. The well becomes a short circuit. After steam stimulation, the usual practice is to produce without stimulation until further stimulation is necessitated or production terminated.
Obviously, in the steam flooding technique distillation plays only a modest role at best for very heavy crudes such as the Peace River bitumens because they do not contain any considerable light values. Consequently, the action of steam in stimulating recovery from deposits such as the Peace River bitumens must be by viscosity reduction from heating, thermal expansion of the formation, and the driving force of the steam. Even then, recovery can be modest because of channeling resulting from the permeability of the deposits, fractures, and gravity override between the steam and liquid in the hot and cold zones.
Importantly also, is the effect of even modest distillation on bitumens or tars. With these crudes, the boiling away of lights will cause the residual crude to become so viscous that no further recovery would be possible, even with the viscosity lowering effect of high temperature from the steam.
Consequently, it has been thought that steam drive recovery is limited to deposits with an API gravity of 20.degree. or greater.
Cold solvent stimulation of oil deposits has improved recovery. Solvents can repair organic and inorganic damage, clean deposited asphaltenes and waxes out from around well bores, and lower the viscosity of the hydrocarbons in the deposit by cutting and demulsification. Demulsification reduces the viscosity of the hydrocarbon deposit because emulsions of water-in-oil and oil-in-water have higher viscosities than oil alone. Solvent stimulation also removes asphaltenes from the deposits. Removal of asphaltenes is especially good with aromatics.
The removal of crude oil from the deposit, however, can create a situation where the solubility of remaining asphaltenes is reduced. Remaining asphaltenes precipitate on surfaces of the deposit and block the passage of crude. Accordingly, to prevent asphaltene precipitation and blockage of the deposit, surfactants have been added to maintain the wetability of deposit surfaces, which prevents blockage.
One of the major drawbacks of solvent stimulation is the high cost of solvent. Quite obviously, if the cost of solvent required to produce effective stimulation of a deposit becomes too great, then solvent stimulation cannot be practiced. Heretofore it has been the practice to produce at least most of the solvents away from the stimulation site. This is so especially with aromatic solvents which are very useful in dissolving asphaltenes.